1. Field of the Invention
This invention relates to a method and apparatus for liquefying natural gas. In another aspect, the invention concerns an improved liquified natural gas (LNG) facility employing a semi-closed loop methane refrigeration cycle.
2. Description of the Prior Art
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume of the natural gas by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
Natural gas is frequently transported by pipeline from the supply source to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when demand exceeds supply. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to −240° F. to −260° F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by indirect heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or combinations of the preceding refrigerants (e.g., mixed refrigerant systems).
In the past, many conventional LNG facilities have used a methane refrigeration cycle (i.e., a refrigeration cycle employing a predominately methane refrigerant) as the final refrigeration cycle for liquefying natural gas. Some conventional LNG facilities utilize an open-loop methane refrigeration cycle, while others use a closed-loop methane refrigeration cycle. In a closed-loop methane refrigeration cycle, the predominately methane refrigerant is not derived from or combined with the natural gas stream being liquefied. In an open loop methane refrigeration cycle, the predominately methane refrigerant is derived from the natural gas undergoing liquefaction, and at least part of the predominately methane refrigerant is recombined with the natural gas stream undergoing liquefaction.
Conventional open-loop and closed-loop methane refrigeration cycles each have their own unique advantages and disadvantages. One disadvantage of conventional closed-loop systems is that a fuel gas compressor is required to compress fuel gas used to power the drivers (e.g., gas turbines) that drive the main refrigerant compressors. Another disadvantage of closed-loop systems is that most closed-loop systems produce an excess of fuel gas that is simply flared from the system. These fuel gas-related problems of closed-loop systems are not common to open-loop systems. However, open-loop systems have their own unique disadvantages. For example, most open-loop systems require the natural gas stream entering the open loop refrigeration cycle to be fully condensed. Further, in open-loop LNG facilities utilizing a demethanizer column for processing the heavies stream discharged from the bottom of the main heavies removal column, the overheads stream from the demethanizer column must be combined with the predominately methane refrigerant and/or compressed because of the pressure difference between the overheads stream from the debutanizer and the overheads stream from the heavies removal column.
Accordingly there is a need for an LNG facility that employs a hybrid methane refrigeration cycle that avoids the disadvantages of both closed-loop and open-loop systems, while still providing the various benefits of closed-loop and open-loop systems.